1. Field of the Invention
The present invention relates to a printed circuit board.
2. Description of the Background Art
An actuator is used in a drive such as a hard disk drive. Such an actuator includes an arm arranged rotatably with respect to a rotation shaft and a suspension board used for a magnetic head that is attached to the arm. The suspension board is a printed circuit board for aligning the magnetic head with a desired track of a magnetic disk.
FIG. 6 is a vertical sectional view showing one example of a conventional suspension board. In the suspension board 900 of FIG. 6, an insulating layer 903 is formed on a metal substrate 902. A pair of write conductors W1, W2 and a pair of read conductors R1, R2 are formed so as to align in sequence on the insulating layer 903.
One ends of the conductors W1, W2, R1, R2 are connected to a magnetic head (not shown). The other ends of the conductors W1, W2, R1, R2 are electrically connected to a write electrical circuit (not shown) and a read electrical circuit (not shown), respectively.
When a write current flows through the write conductors W1, W2, induced electromotive forces are generated in the read conductors R1, R2 by electromagnetic induction in the suspension board 900.
Here, the distance between the write conductors W1, W2 and the read conductor R1 is smaller than the distance between the write conductors W1, W2 and the read conductor R2. This causes a difference in the induced electromotive forces generated in the read conductors R1, R2. As a result, a current flows through the read conductors R1, R2. That is, a crosstalk occurs between the write conductors W1, W2 and the read conductors R1, R2.
Therefore, JP 2004-133988 A proposes a printed circuit board shown in FIG. 7 for preventing occurrence of the crosstalk between the write conductors W1, W2 and the read conductors R1, R2.
FIG. 7 is a vertical sectional view showing another example of the conventional suspension board. In the suspension board 910, a first insulating layer 904 is formed on the metal substrate 902. The write conductor W2 and the read conductor R2 are formed so as to be spaced apart from each other by a distance L1 on the first insulating layer 904.
A second insulating layer 905 is formed on the first insulating layer 904 so as to cover the write conductor W2 and the read conductor R2. On the second insulating layer 905, the write conductor W1 is formed at a position above the read conductor R2, and the read conductor R1 is formed at a position above the write conductor W2.
The distance between the read conductor R1 and the write conductor W2 that are positioned one above the other and the distance between the read conductor R2 and the write conductor W1 that are positioned one above the other are L2, respectively.
In the suspension board 910 of FIG. 7 having the foregoing configuration, the distances between the write conductors W1, W2 and the read conductor R1 are substantially equal to the distances between the write conductors W1, W2 and the read conductor R2, respectively. Accordingly, it is considered that the magnitudes of the induced electromotive forces generated in the read conductors R1, R2 are substantially equal when the write current flows through the write conductors W1, W2.
In the suspension boards 900, 910 shown in FIGS. 6 and 7, impedances of the conductors W1, W2, R1, R2 vary depending on the magnitudes of coupling capacitances between the conductors W1, W2, R1, R2 and the metal substrate 902.
Here, the distance between the write conductor W1 and the metal substrate 902 is different from the distance between the write conductor W2 and the metal substrate 902 in the suspension board 910 of FIG. 7. Moreover, the distance between the read conductor R1 and the metal substrate 902 is different from the distance between the read conductor R2 and the metal substrate 902.
In this case, the coupling capacitance between the write conductor W1 and the metal substrate 902 is different from the coupling capacitance between the write conductor W2 and the metal substrate 902. Moreover, the coupling capacitance between the read conductor R1 and the metal substrate 902 is different from the coupling capacitance between the read conductor R2 and the metal substrate 902.
Therefore, a difference occurs in the impedances of the write conductor W1 and the write conductor W2, and a difference occurs in the impedances of the read conductor R1 and the read conductor R2 in the configuration of the suspension board 910. This may cause a transmission error of a differential signal through the write conductors W1, W2 and cause a transmission error of a differential signal through the read conductors R1, R2.